In several reviews, Wilchek and Bayer have described many biotin containing reagents which have enhanced the versatility and utility of the avidin-biotin complex in studying biological interactions and accomplishing separations. Methods in Enzymology, Vol. 184[13], pp. 123-138, titled "Biotin-Containing Reagents" and Analytical Biochemistry, Vol. 171, pp. 1-32 (1988), titled The Avidin-Biotin Complex in Bioanalytical Applications. As observed in the first of these reviews, such reagents can be used for biotin labeling of a variety of sites (e.g., amino, thiols, imidazoles and phenols) on targets, binders and/or probes. The review also states that photoreactive reagents for indiscriminate labeling have been described. Wedelund, et al. describe a trifunctional biotin reagent containing the photo-reactive phenyl azide moiety for photoaffinity labeling. Biol. Chem. Hoppe-Seyler, Vol. 370 pp. 251-158 (March 1989). A reagent for covalently attaching biotin to proteins via a cleavable connector arm has also been described. Mouton, et al., Archives of Biochemistry and Biophysics, Vol. 218, No. 1, pp. 101-108 (1982).